Alrighty, so I had a sorority with four girls that was going along just great--pecking orders were established without too much trouble, no one was picking fights, all that stuff. I decided to introduce two new girls, just to get up to that ideal six, and did the whole nine yards of rearranging everything before reintroducing them one at a time. I stayed at home for about four hours after the last girl was added, and things were going great, so I didn't feel worried about stepping out for a few hours to go to a movie. Upon returning, I discovered that all hell had broken loose while I was gone.

Haru, formerly the low (wo)man on the totem pole, has taken over the tank. Kat, who once ruled over the other three girls with complete authority, has had two chunks taken out of her tail (she was so, so beautiful) and is in hiding. The problem doesn't seem to be the new girls, as they both generally mind their own business and don't show any real damage besides one that has a small nip out of her tail. The two other girls from the original four (Sparrow and Nanaka) are also doing well. So I guess my question is... How common is this sort of drastic behavior change, and should I pull Haru out now for a time-out or see if the issues have been settled? The girls have plenty of hiding places, and I've added plants to the tank, so that shouldn't be the problem. I just don't want to wake up tomorrow and discover that one of the girls is gone.

If there are plenty of hiding spaces, I would say leave Haru in for another day or so, usually aggresion calms down after a day or so with the occasional spat once in a while. I recently added a female to my sorority and not only are there fights between her and the dominant, but the some of the other ones who were timid before have decided to wage a campaign for the top spot. Perhaps it's happening because the new bettas pose a potential threat to the dominant one, so the lower ranking ones see this as a chance to raise their rank, possibly even be dominant. Well now that I've rambled on about animal hierarchy, I hope this helps you. Just keep an eye on them and if you think it's getting to be too much, then remove the one causing chaos for a day or so then reintroduce them.

Cool, thank you! And I appreciate the ramblings on animal hierarchy. I've dealt with pecking orders amongst dogs, chickens, and rats before, but never fish, and they're quite different. I've been watching them, and everything seems to be settling down--no one's chasing anyone else around the top nonstop or anything--so I'm hoping everything's settled down. It was just weird to come home, turn on the lights in my room, and discover that something had happened when it had all been so calm the whole time I was there.

Closer analysis reveals that Haru is not #1, but #2. Nanaka is alpha, but I guess Haru is just out to prove herself. It was like this before, too; Kat was alpha, but Sparrow was the one picking on everyone besides Kat, who could scare off anyone with a little flare.

Awesome, I'm glad everything is working out for your sorority! I'm actually having trouble figuring out who's the dominant in my tank. It might actually be the smallest betta in the tank, she never seems to be chased and is always doing the chasing. :P

Nearly all fishes from Tetras to Cichlids will establish pecking order and dominant fish will prevail over fishes they perceive to be weaker.Young fishes often get along well while juveniles ,but as they grow and become sexually mature, they may not always get along.
Fish that are by nature territorial,,, will often take issue with new tank mates. This applies to dominant fish in the tank as well as sub-dominant fish.
While personality may play a part, Genetics,and natural instincts,must not be overlooked.
Dynamics in the aquarium are always changing.

Well, I ended up pulling Haru out of the tank today. After terrorizing Kat to the point that she wouldn't even come out to eat, Haru decided that Sparrow was her next target and I wasn't going to have anyone else beaten up like poor Kat. She's been out of the tank for about seven hours now, and though I realize that's not enough time for everyone to adjust, the tank has been infinitely calmer. The two top girls will flare up and chase the others out of their territory, but not around and around the tank like Haru used to do. I'm going to try and add her back in after a few days, but I'll be paying very close attention. She may get to live out the rest of her days in a one-gallon bowl with a snail for company.